Alternative titles; symbols
HGNC Approved Gene Symbol: INSIG1
Cytogenetic location: 7q36.3 Genomic coordinates (GRCh38): 7:155,297,878-155,310,235 (from NCBI)
By subtractive hybridization and differential screening following induction by insulin, Diamond et al. (1993) identified a 'delayed-early gene,' designated CL-6, from the rat H35 cell line. The deduced 256-amino acid rat protein is highly hydrophobic. CL-6 is the most highly insulin-induced gene in H35 cells. Northern blot analysis showed that CL-6 is expressed in normal liver and kidney as well as in regenerating liver. Peng et al. (1997) isolated a human gene that they designated insulin-induced gene-1 (INSIG1). They found it to share 80% identity with the rat CL-6 gene within the translated region.
By PCR of a HepG2 cell cDNA library, Yang et al. (2002) obtained a cDNA encoding INSIG1. The deduced 277-amino acid protein has at least 6 membrane spanning regions and differs from the sequence reported by Peng et al. (1997) at residues 27 (ala vs thr), 31 (ala vs pro), 32 (ala vs pro), 99 (ala vs thr), 170 (val vs gly), and 172 (val vs gly). The human INSIG1 protein is 84.6% and 84.2% identical to rat and mouse Insig1, respectively. Northern blot analysis detected expression of INSIG1 in all tissues tested, with high expression in liver. Immunofluorescence of epitope-tagged INSIG1 in stably transfected CHO cells showed an endoplasmic reticulum (ER) localization when the cells were cultured in either the absence or presence of sterols.
Using coimmunoprecipitation and tandem mass spectrometry, Yang et al. (2002) identified INSIG1 as an ER protein that binds the sterol-sensing domain of SREBP (see 184756) cleavage-activating protein (SCAP; 601510) and facilitates retention of the SCAP/SREBP complex in the ER. In sterol-depleted cells, SCAP escorts SREBPs from ER to Golgi for proteolytic processing, thereby allowing SREBPs to stimulate cholesterol synthesis. Using blue native-PAGE, Yang et al. (2002) showed that sterols induce binding of SCAP to INSIG1, and this binding was correlated with the inhibition of SCAP exit from the ER. Overexpression of INSIG1 increased the sensitivity of cells to sterol-mediated inhibition of SREBP processing. Mutant SCAP (tyr298 to cys) failed to bind INSIG1 and was resistant to sterol-mediated inhibition of ER exit. The authors concluded that by facilitating sterol-dependent ER retention of SCAP, INSIG1 plays a central role in cholesterol homeostasis.
Sever et al. (2003) showed that degradation of HMG-CoA reductase (142910) is accelerated by the sterol-induced binding of its sterol-sensing domain to the ER protein INSIG1. Accelerated degradation was inhibited by overexpression of the sterol-sensing domain of SCAP, suggesting that both proteins bind to the same site on INSIG1. Whereas INSIG1 binding to SCAP led to ER retention, INSIG1 binding to HMG-CoA reductase led to accelerated degradation that could be blocked by proteasome inhibitors. The authors concluded that INSIG1 plays an essential role in the sterol-mediated trafficking of HMG-CoA reductase and SCAP.
Song et al. (2005) found that rodent Gp78 (AMFR; 603243), a membrane-bound E3 ubiquitin ligase, associated with Insig1. Insig1 bound the membrane domain of Gp78 in the absence or presence of sterols, and upon addition of sterols, HMG-CoA reductase was recruited to the complex. Knockdown of Gp78 by RNA interference prevented sterol-dependent ubiquitination and degradation of endogenous reductase. Vcp (601023), an ATPase that participates in postubiquitination steps of ER-associated degradation and is required for reductase degradation, indirectly associated with Insig1 by binding Gp78. The results identified GP78 as a ubiquitin ligase that initiates sterol-dependent degradation of HMG-CoA reductase, and INSIG1 as the bridge between GP78/VCP and the reductase substrate.
Li et al. (2003) presented experimental results suggesting that INSIG1 expression restricts lipogenesis in mature adipocytes and blocks differentiation in preadipocytes. They examined gene expression in the fat tissue of normal mice at the onset of diet-induced obesity. INSIG1 mRNA rose progressively with a high-fat diet and declined on a restricted diet. Transfection of mouse or human INSIG1 into 3T3-L1 preadipocytes completely prevented oil red O staining and blocked upregulation of adipocyte fatty acid-binding protein-2 (FABP2; 134640), peroxisome proliferator-activated receptor gamma-2 (PPARG2; see 601487), and carbohydrate response element-binding protein, while reducing downregulation of preadipocyte factor-1 (PREF1; 176290).
Asp205 in INSIG1 and asp149 in INSIG2 (608660) are conserved residues that abut the fourth transmembrane helix at the cytosolic side of the ER membrane. Gong et al. (2006) found that mutation of these residues to alanine resulted in INSIG proteins that were unable to bind SCAP and suppress cleavage of SREBPs. The mutant INSIGs were also ineffective in accelerating sterol-stimulated degradation of HMG CoA reductase.
Xu et al. (2020) showed that activated AKT (AKT1; 164730) in human hepatocellular carcinoma cells phosphorylates cytosolic phosphoenolpyruvate carboxykinase-1 (PCK1; 614168), the rate-limiting enzyme in gluconeogenesis, at ser90. Phosphorylated PCK1 translocates to the endoplasmic reticulum, where it uses GTP as a phosphate donor to phosphorylate INSIG1 at ser207 and INSIG2 at ser151. This phosphorylation reduces the binding of sterols to INSIG1 and INSIG2 and disrupts the interaction between INSIG proteins and SCAP, leading to the translocation of the SCAP-SREBP complex to the Golgi apparatus, the activation of SREBP proteins (SREBP1, 184756 or SREBP2, 600481) and the transcription of downstream lipogenesis-related genes, proliferation of tumor cells, and tumorigenesis in mice. In addition, phosphorylation of PCK1 at ser90, INSIG1 at ser207, and INSIG2 at ser151 was not only positively correlated with the nuclear accumulation of SREBP1 in samples from patients with hepatocellular carcinoma, but also associated with poor hepatocellular carcinoma prognosis.
By fluorescence in situ hybridization, Peng et al. (1997) mapped the INSIG1 gene to chromosome 7q36.
Takaishi et al. (2004) infected Zucker diabetic fatty rats (see 601007) with recombinant adenovirus containing Insig1 or Insig2 (608660) cDNA. Triacylglycerols in the liver and plasma of control diabetic rats rose steeply, whereas the Insig-infected rats exhibited substantial attenuation of hepatic steatosis and hyperlipidemia. Insig overexpression was also associated with a reduction in the elevated level of nuclear Srebp1c and reduced expression of Srebp1c lipogenic target enzymes. In normal animals, overexpression of the Insigs reduced the increase in Srebp1c mRNA and its target enzymes caused by refeeding. Takaishi et al. (2004) concluded that both Insigs have antilipogenic action.
Engelking et al. (2005) found that, whereas cholesterol feeding reduced nuclear Srebps and lipogenic mRNAs in wildtype mice, this feedback response was severely blunted in Insig1/Insig2 double-knockout mice, and synthesis of cholesterol and fatty acids was not repressed.
In cortical neuron culture, Taghibiglou et al. (2009) found that activation of NMDA receptors resulted in increased activation and nuclear accumulation of SREBP1. The activation was primarily mediated by the NR2B (138252) subunit-containing receptor. Inhibition of NMDAR-dependent SREBP1 activation by cholesterol decreased NMDA-induced excitotoxic cell death. Similarly, shRNA against SREBP1 also resulted in decreased cell death in culture. These findings implicated SREBP1 as a mediator of NMDA-induced excitotoxicity. NMDAR-mediated activation of SREBP1 was shown to result from increased Insig1 degradation, which could be inhibited with an interference peptide. In a rat model of focal ischemic stroke, systemic administration of the INSIG1 interference peptide prevented SREBP1 activation, substantially reduced neuronal damage, and improved behavioral outcome.
Diamond, R. H., Du, K., Lee, V. M., Mohn, K. L., Haber, B. A., Tewari, D. S., Taub, R. Novel delayed-early and highly insulin-induced growth response genes: identification of HRS, a potential regulator of alternative pre-mRNA splicing. J. Biol. Chem. 268: 15185-15192, 1993. [PubMed: 7686911]
Engelking, L. J., Liang, G., Hammer, R. E., Takaishi, K., Kuriyama, H., Evers, B. M., Li, W.-P., Horton, J. D., Goldstein, J. L., Brown, M. S. Schoenheimer effect explained--feedback regulation of cholesterol synthesis in mice mediated by Insig proteins. J. Clin. Invest. 115: 2489-2498, 2005. [PubMed: 16100574] [Full Text: https://doi.org/10.1172/JCI25614]
Gong, Y., Lee, J. N., Brown, M. S., Goldstein, J. L., Ye, J. Juxtamembranous aspartic acid in Insig-1 and Insig-2 is required for cholesterol homeostasis. Proc. Nat. Acad. Sci. 103: 6154-6159, 2006. [PubMed: 16606821] [Full Text: https://doi.org/10.1073/pnas.0601923103]
Li, J., Takaishi, K., Cook, W., McCorkle, S. K., Unger, R. H. Insig-1 brakes lipogenesis in adipocytes and inhibits differentiation of preadipocytes. Proc. Nat. Acad. Sci. 100: 9476-9481, 2003. [PubMed: 12869692] [Full Text: https://doi.org/10.1073/pnas.1133426100]
Peng, Y., Schwarz, E. J., Lazar, M. A., Genin, A., Spinner, N. B., Taub, R. Cloning, human chromosomal assignment, and adipose and hepatic expression of the CL-6/INSIG1 gene. Genomics 43: 278-284, 1997. [PubMed: 9268630] [Full Text: https://doi.org/10.1006/geno.1997.4821]
Sever, N., Yang, T., Brown, M. S., Goldstein, J. L., DeBose-Boyd, R. A. Accelerated degradation of HMG CoA reductase mediated by binding of insig-1 to its sterol-sensing domain. Molec. Cell 11: 25-33, 2003. [PubMed: 12535518] [Full Text: https://doi.org/10.1016/s1097-2765(02)00822-5]
Song, B.-L., Sever, N., DeBose-Boyd, R. A. Gp78, a membrane-anchored ubiquitin ligase, associates with Insig-1 and couples sterol-regulated ubiquitination to degradation of HMG CoA reductase. Molec. Cell 19: 829-840, 2005. [PubMed: 16168377] [Full Text: https://doi.org/10.1016/j.molcel.2005.08.009]
Taghibiglou, C., Martin, H. G. S., Lai, T. W., Cho, T., Prasad, S., Kojic, L., Lu, J., Liu, Y., Lo, E., Zhang, S., Wu, J. Z. Z., Li, Y. P., Wen, Y. H., Imm, J.-H., Cynader, M. S., Wang, Y. T. Role of NMDA receptor-dependent activation of SREBP1 in excitotoxic and ischemic neuronal injuries. Nature Med. 15: 1399-1406, 2009. [PubMed: 19966780] [Full Text: https://doi.org/10.1038/nm.2064]
Takaishi, K., Duplomb, L., Wang, M.-Y., Li, J., Unger, R. H. Hepatic insig-1 or -2 overexpression reduces lipogenesis in obese Zucker diabetic fatty rats and in fasted/refed normal rats. Proc. Nat. Acad. Sci. 101: 7106-7111, 2004. [PubMed: 15096598] [Full Text: https://doi.org/10.1073/pnas.0401715101]
Xu, D., Wang, Z., Xia, Y., Shao, F., Xia, W., Wei, Y., Li, X., Qian, X., Lee, J.-H., Du, L., Zheng, Y., Lv, G., Leu, J., Wang, H., Xing, D., Liang, T., Hung, M.-C., Lu, Z. The gluconeogenic enzyme PCK1 phosphorylates INSIG1/2 for lipogenesis. Nature 580: 530-535, 2020. [PubMed: 32322062] [Full Text: https://doi.org/10.1038/s41586-020-2183-2]
Yang, T., Espenshade, P. J., Wright, M. E., Yabe, D., Gong, Y., Aebersold, R., Goldstein, J. L., Brown, M. S. Crucial step in cholesterol homeostasis: sterols promote binding of SCAP to INSIG-1, a membrane protein that facilitates retention of SREBPs in ER. Cell 110: 489-500, 2002. [PubMed: 12202038] [Full Text: https://doi.org/10.1016/s0092-8674(02)00872-3]